PDB is an ancient disease, claimed to be identified in a Neanderthal skull;

- Is a common disease (5% prevalence in UK) but most are asymptomatic.
- Primarily a disorder of increased osteoclastic resorption with secondary stimulation of osteoblasts.
- Pathophysiology: In Paget's disease, the osteoclasts look 'bizzare' on histology AND they have increased sensitivity to vitamin D.
- Pathogenesis incompletely understood.
- #autosomalDominant with incomplete penetrance.
- Commoner in older males (onset usually after 55)
- Thickening of skull bones. -> can lead to hearing loss, increase in hat size
- Cotton wool appearance on skull X ray.
- Increased uptake at lesions is seen on bone scintigraphy.
- Calcium and phophate are normal
- Bone: Fractures, Bone sarcoma, Deafness (cranial nerve entrapment)
Spondyloarthritis (SpA)
- radiographic changes in the sacroiliac joints in patients with axial SpA
- and asymmetric (oligo)arthritis, heel enthesitis and dactylitis in patients with peripheral SpA
- Rheumatoid factors negative. (<- Seronegative arthropathies)
Inflammatory osteoproliferative lesions in the spine are called syndesmophytes (marginal and non-marginal), and degenerative osteoproliferative lesions are called osteophytes. Syndesmophytes are more vertically oriented than osteophytes.

| Arise from calcification withn ligaments |
Arise from bone (vertebral bodies) |
| Seen in inflammatory (spondylo) arthropathies |
See in Osteoarthritis |
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[!TIP] Mnemonic: RePsAnkIb
- Reactive
- Psoriatic
- Anky spondylos
- IBD associated
Ankylosing = stiffness or fixation of a joint by disease

- Advanced cases: fusion of the sacroiliac joints, and bamboo spine. (Gadolinum MRI is more sensitive than Xray at detecting early changes).
Diagnosis
"Plain x-ray of the sacroiliac joints is the most useful investigation in establishing the diagnosis."
- sacroiliitis is usually the first manifestation 5 and is symmetrical and bilateral
- the sacroiliac joints first widen before they narrow
- subchondral erosions, sclerosis, and proliferation on the iliac side of the SI joints
- at end-stage, the SI joint may be seen as a thin line or not visible
The sacroiliac joint is actually a synovial joint with hyaline on the sacral surface and fibrous cartilage on the iliac surface Source.
- Prevention of syndesmophyte formation -> early diagnosis and initiation of exercise regimen.
- TNF-α blockers - etanercept and adalimumab provide HUGE symptomatic relief. They reduced morbidity
Bone changes seen in various diseases

- Acute, asymmetrical, lower limb oligoarthritis days to weeks after infection such as dysentery or some sexually acquired infections.
- keratoderma blennorhagica - raised red plaques and pustules. Can occur on soles


- Majority of patients recover in 6 months but 30% develop chronic symptoms.
- Management is symptomatic. (NSAIDS, corticosteroids, DMARDS like sulfasalazine and infliximab if NSAIDS don't work or a contraindicated)
Langerhans cell histiocytosis
- A multisystem disease
- Birbeck granules present on EM: They have a "tennis racket" shape.
- More common in children (1-3 years of age) but can present in any age even in adults.
- Is one cause of interstitial lung disease.
- Adults presents with pulmonary involvement.
- The disease involves the formation of multiple granulomas throughout the body.
- Adults commonly have lytic lesions in the jaw and skull.
- Pulmonary involvement: also commoner in adults. non productive cough, spontaneous pneumothorax, chest pain etc.
- Upper and middle zone involvement is characteristic.
Other features:
- bone pain, typically in the skull or proximal femur
- cutaneous nodules
- recurrent otitis media/mastoiditis
